If the belt is off or slack put it back on and tighten it up. This will give some resistance so that when you wack the wrench with a hammer it should loosen the nut. If the wrench is too fat to easily fit between the pulley bolt heads and the clutch you may be hitting the pulley bolts when you wack the wrench. You may have to grind the wrench down with a bench grinder so that it fits better.